Some integrated circuits are designed for use in generating variable frequency tuning signals for radio frequency (RF) devices such as cellular telephones, global positioning system (GPS) receivers, pagers, and the like. These devices are known generally as RF synthesizers. Wireless communication systems typically require frequency synthesis in both the receive path circuitry and the transmit path circuitry. For example, cellular phone standards in the United States and Europe define a cellular telephone system with communication centered in two frequency bands at about 900 MHz and 1800 MHz.
RF synthesizers use phase lock loops (PLLs) to synthesize the necessary tuning frequencies. The values of the loop dividers and reference dividers are programmable to generate different output frequencies to allow different channels to be tuned. Each PLL includes a voltage controlled oscillator formed by an inductance-capacitance (LC) oscillator. Since the VCO output frequency depends on the LC product, the output frequency can be controlled by adjusting the value of a variable capacitance.
Though the frequency synthesizer may be generally contained within an integrated circuit package, VCO inductors used for band selection purposes may be “off-package” or circuit board-mounted, and are connected to other package-contained PLL circuitry to form a frequency synthesizer circuit by means of contacts or pin connections. Off-package inductors are often mounted on a circuit board by the manufacturer.
With the higher-frequency bands now being used for cellular telephones and other similar applications, the values required for external inductors have been reduced and may be, for example, on the order of tenths of nanoHenrys (nH). These smaller inductances allow the off-chip inductors to be made more simply by using short traces on the printed circuit board between two integrated circuit pins rather than by discrete inductors. However these values are becoming so small that undesirable parasitic effects can no longer be ignored.
These parasitic effects are amplified during manufacturing test, when instead of being permanently mounted to a printed circuit board, the integrated circuit is temporarily inserted into a test socket while the pins are driven and/or sampled by an integrated circuit tester. The printed circuit board inductance can be simulated during manufacturing test by adding a similar trace on the test board. However a sufficiently small inductance within a narrow range of values has been difficult to achieve using known test hardware. What is needed then is a test apparatus that provides an external inductor having an inductance falling within a desirably narrow range of values. This and other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.